1. Field of the Invention
The present invention relates to method and apparatus for cooling molded plastic pieces after the molding is finished. In particular, the present invention relates to method and apparatus for post mold cooling of plastic pieces wherein at least two cooling stations are provided to cool the interior of the plastic pieces. The present invention also relates to method and apparatus wherein pressurized sealing of each plastic piece is carried out at a cooling station such that the pressure seal does not contact the plastic piece. The present invention also relates to method and apparatus wherein plastic piece cavity tubes are provided with vents to prevent deformation of negative draft plastic pieces. Preferably, the plastic pieces comprise plastic preforms.
2. Related Art
A variety of post mold cooling methods are currently employed on injection molding machines (e.g., an Index machine platform) in various sequences to optimize the cooling of freshly molded plastic parts. Some parts (for example plastic preforms) are typically injection-molded using PET resin, and can have wall thicknesses varying from about 2.00 mm to greater than 4.00 mm, and consequently require extended cooling periods to solidify into defect-free parts. Heavy walled parts (such as these made from a material that has a high resistance to thermal heat transfer, like plastic resin) can exhibit a “reheating” phenomena that can produce defective parts after they have been ejected from the mold.
In the case of PET performs some of these manufacturing defects are:                Crystallinity: The resin recrystallizes due to the elevated temperature of the core resin not cooling quickly enough. The white appearance of the crystals impairs the clarity of the final product.        Surface blemishes: The ejected performs, initially having solidified surfaces are reheated by the core material which causes the surface to soften and be easily marred. Sometimes this surface reheating can be severe enough to cause touching parts to weld together.        Geometric inaccuracies: Handling partly-cooled performs or attempting to further cool them in devices that do not maintain their geometric shape while their surfaces are reheated can cause the preform's round diameter to become oval shaped or the smooth surface to become wrinkled or non-linear.        
The above-noted problems could be alleviated somewhat by extending the cooling time of the injection molded performs. However, this will cause the injection molding cycle to be lengthy, typically 25 seconds or longer, wherein the majority of this time was used solely for cooling purposes. In an effort to improve the production efficiency of this process, several techniques are employed to perform a “post mold cooling” function, wherein partially-cooled performs are ejected from the injection mold after an initially cooled surface skin has formed to allow the part to be ejected without deformation. The partially-cooled performs are then handed off to a downstream device that continues to hold the perform while removing the remaining heat so that the preform can subsequently be handled without damage. Typically, the preform surface temperature needs to be lowered to about 72° C. to ensure safe handling.
The early ejection of partially-cooled performs released the injection molding equipment earlier in the molding cycle, thereby significantly improving the production efficiency of the equipment. Injection molding cycle times typically were halved from 25 seconds to about 12 seconds or less in some instances depending on the perform design being molded.
Some examples of post mold cooling technology are shown in U.S. Pat. Nos. 4,729,732; Re. 33,237; 5,447,426; and 6,171,541, the contents of each being incorporated herein by reference.
Another approach to extending the cooling time for performs is to utilize a turret molding machine in which more than one set of injection molding cores are employed. An example is the Index machine, shown in U.S. Pat. Nos. 5,728,409; 5,830,404; 5,750,162; and 5,817,345 (the contents of each being incorporated herein by reference), which teach using a turret block having four faces and four core sets that are sequentially mated with one cavity set to hold the injection mold performs. Preforms molded on this kind of equipment can be produced in molding cycle times of typically 10-13 seconds.
A disadvantage of the above-described approach is the cost of the additional core side tooling that is required. In order to reduce this cost, Index machines with fewer core side tooling sets were employed. However, to maintain the cycle times, additional post mold cooling devices are needed to complete the perform cooling. Examples of Index machines with post mold cooling devices are shown in U.S. Pat. Nos. 6,059,557; 6,123,538; 6,143,225; and 6,113,834, the contents of each being incorporated herein by reference.
One technique for improving the rate of heat transfer from a cooling perform is to pressurize its interior volume while cooling it in a cavity. This method helps keep the preform's exterior surface in contact with the cooling cavity surface, and counters the shrinkage of the perform which tends to separate the two surfaces. This allows good heat transfer to be maintained. Examples of pressurized perform cooling are shown in U.S. Pat. Nos. 4,950,152; and 5,679,306, and in EP 0 900 135, the contents of each being incorporated herein by reference.
Therefore, there is a need for post-mold cooling method and apparatus, which provides rapid, efficient cooling while further reducing the molding cycle time to further decrease the cost of producing molded plastic pieces.